Distributed Base Stations are based on an architecture in which the radio function unit, also known as the remote radio head (RRH), is physically separated from the digital function unit, or baseband unit (BBU). For instance, the RRH can be installed on the top of tower close to the antenna, thereby reducing the loss when compared to traditional base stations in which RF signals have to travel through a long cable from the base station cabinet to the antenna.
C-RAN (Cloud or Centralized radio access network) is an architectural evolution of the distributed base station that allows BBUs to be significantly further away from the RRH, thereby enabling large scale centralised base station deployment.
3GPP has defined various L1 PHY and L2 functions for 4G LTE, along with protocols for the handling of information among the layers. However, the deployment architecture for realizing these functions in a physical system, which includes the fronthaul (FH) connection between the centralized baseband processing and RRH, is not specified. This flexibility means that not all baseband processing functions must necessarily be centralized away from the RRH, and instead the baseband processing functions may be divided between a centralised location and co-location with the RRH. The same is true for 5G networks.